Organic polyisocyanates are utilized hitherto for various applications as foams, elastomers, paints, adhesives and the like by the reaction with active hydrogen compounds.
Such an organic polyisocyanate can also be used by the modification to a polyisocyanate compound having an isocyanurate ring.
The product prepared by reaction of the modified polyisocyanate compound with an active hydrogen compound has heat resistance, flame resistance, rigidity, toughness and the like which are characteristic of isocyanurates, and is utilized in applications where such properties are required.
Accordingly, a variety of isocyanuration polymerization is conducted for isocyanuration of organic polyisocyanates. (The term "isocyanuration" means hereinafter a trimerization of an isocyanate compound to form an isocyanurate ring.) For example, Japanese Patent Laid-open Publications Sho. 54-32490 and Sho. 52-69497 respectively describe methods for the isocyanuration by use of a catalyst such as a metal salt of acetylacetone, an alkali metal salt of an organic acid, and the like. These production processes are not satisfactory, because the selectivity of the catalyst is very narrow for the organic polyisocyanate; and particularly in the isocyanuration polymerization, an organic isocyanate in which the difference between the reactivities of the first and the second isocyanate groups is little like those in diphenylmethane diisocyanate is liable to polymerize to become viscous and to give a partially non-uniform product having a low isocyanate content and low compatibility with other resins.
Further, upon carrying inactivation of the catalyst in order to improve stability of the modified polyisocyanate, if the reaction is conducted without a solvent insoluble precipitate tends to be formed and it is difficult to control the isocyanuration reaction, so that it is not satisfactory industrially and improvement thereof has been desired. In other words, a modified organic polyisocyanate having an isocyanurate ring has been desired which is prepared readily through isocyanuration of an organic polyisocyanate, and which is stable in storage, and excellent in compatibility to other resins.
Heretofore, the preparation of a polyisocyanurate foam having flame resistance and low smoking property has been carried out in the presence of a trimerization catalyst upon a reaction of a polyisocyanate with an active hydrogen compound. However, further improvement of the properties are desired.
A polymethylenepolyphenyl-polyisocyanate (hereinafter referred to as "polymeric MDI") exhibits heat resistance based on its characteristic feature when reacted with polyols.
Polymeric MDI solely or a polyol adduct of polymeric MDI is used nowadays. However, polyol adducts are liable to deteriorate by heat, and are limited in use for flame-resistant foams.
Polyisocyanurate foams are widely used for heat insulating materials, light-weight structural materials, and sound absorption materials because of their superior characteristics. The introduction of the isocyanurate ring into polyurethane crossliking makes heat resistance, hydrolysis resistance, and dimension stability higher. However, the application fields thereof is limited because of an increase of a smoking quantity in combustion on heating to a high temperature.
For the improvement of the disadvantage, an additive such as ammonium polyphosphate, a phosphate ester, p-nitroaniline sulfonic acid, and the like is added to accelerate carbonization of foam on contact with flame to reduce the smoking quantity.
Further, as a method for lowering the smoking properties attributed to control of combustibility by addition of a heat-resistant inorganic powder, for example, addition of calcium carbonate, ammonium phosphate, ammonium sulfate, and the like is tried to dilute a combustible gas generated from the foam with an inert gas (CO.sub.2, NH.sub.3, etc.) generated by pyrolysis of the inorganic powder and to lower combustibility by depression of combustion.
However, since such an additive is required to be added in a large amount, these methods have disadvantages lowering storage stability of reaction mixing liquid component and mechanical properties of the foam.
Further, for imparting high flame resistance to a foam, the concentration of isocyanurate rings in formulation containing a polyisocyanate and a polyol is increased by raising the index (NCO/OH molar ratio) to make the concentration of isocyanurate linkage in the foam high so as to possess heat resistance and flame resistance derived from the isocyanurate linkage. The raising of the NCO/OH ratio, however, is limited because an excessively high ratio leads to a large amount of unreacted isocyanate group contained, resulting in disadvantages of increased smoking in combustion and explosive breakage upon contact with flame, and the foam thus prepared has considerable brittleness coming from the characteristic property of isocyanurate linkage at the same time. Accordingly, sufficient properties are not attained as an semi-incombustible material, and further improvement has been desired. That is, it has been desired to improve the modified polyisocyanate for polyisocyanurate foams, and flame resistance, smoking property, friability, heat resistance, compatibility with resins, etc. for polyisocyanurate foam produced from the polyisocyanate, a polyol, a blowing agent, an additive, etc. upon using as incombustible materials.
The inventors have found after comprehensive investigation that a modified organic polyisocyanate which contains an isocyanurate ring is effectively produced by addition of an organic polyisocyanate trimerization catalyst, a specific additive, optionally a ferrocene compound, etc., and completed the present invention.
The inventors of the present invention has also succeeded in isocyanuration of a polymeric MDI after comprehensive investigation, and also found that the foam prepared by using the isocyanate is highly superior in incombustibility, low smoking property, low friability, etc., and completed the present invention.